Gamma ray assessment of subsurface water rock interaction in abuja from geolo...
FELIX PROJECT
1. 1
CHAPTER ONE
INTRODUCTION
1.1 GENERALSTATEMENT:
Groundwater is one essential but necessary substitute to surface water in
every society. It’s no doubt a hidden; replenish able resource whose occurrence
and distribution greatly varies according to the local as well as regional geology,
Hydrogeological settings and to an extent the nature of human activities on the
hand.
Groundwater occurrence in a Precambrian Basement terrain is hosted within
zones of weathering and fracturing which often are not continuous in vertical and
lateral extent (Jeff, 2006). There is a steady rise in the demand for ground water in
most hard rock areas most of which cannot boast of any constant surface source of
water supply (Adam, 1994). The failure rate in most ground water project recorded
in Basement Complex aquifers has informed the general acceptance of a
geophysical survey as a compulsory prerequisite for any successful water well
drilling project (Dan Hassan, 1999). The electrical resistivity method involving the
vertical electrical sounding (VES) technique is extensively gaining application in
environmental, groundwater and engineering geophysical investigations.
2. 2
Integrating electrical resistivity method of geophysical study with Basement
subsurface structure trends provides a very useful tool in predicting ground water
distributing pattern in a Basement Complex rocks terrains. Vertical electrical
sounding (VES) as a method of geophysical exploration measures the change of
formation resistivity with depth. Based on this analogy, a study was carried out in
Ayaran village in Akoko Edo Local Government Area of Edo State which is within
the Basement Complex terrain of South Western Nigeria and four of such sounding
were conducted to investigate the subsurfacefor borehole sinking.
A first qualitative interpretation of the geo-electric sounding curves gives a general
outline of the geologic settings of the area from which assertion can be made on
the geo-electric layers and the nature of these layers.
Top soil, sedimentary layers, weathering front and basement (massive or fractured)
are typical of Basement Complex terrain and as such water is most likely to be
found in joints and fractures. The electrical methods have proved versatile in
determining such aquifers pertinent in these areas.
3. 3
TYPICAL RESISTIVITYOF SOME EARTH MATERIALS
Dry (Ωm) Wet (Ωm)
Top soil 200-2400 45-250
Dari crust 400-1600 270-380
Clay 1-100
Alluvium & Sand 800-2500 100-800
Highly weathered/
fractured rock
300-106
Massive bedrock 1000-106
Granite >102-106
Shale 10-104
Gabbro 103-106
Schist 10-104
Sandstone 1-108
Fig. 1: shoeing typical resistivity of some earth materials.
4. 4
Fig.2:showing the geologymap of Nigeria
1.2 AIMS AND OBJECTIVES
To establish that ground water development in Basement Complex is facilitated
by proper geophysical investigation prior to drilling.
To provide avenue to get use to VES data acquisition and interpretation.
To show the role of vertical electrical sounding in groundwater exploration in
hard rock areas.
5. 5
To provide information on the existing subsurface layering in the study area for
the purpose of planning and executing successful bore hole drilling
programmes.
To define the nature and distribution of ground water in a typical Basement
Complex aquifers.
1.3 SCOPE OF STUDY
It has been observed that improper subsurface investigation can result in
failure of borehole schemes. With a proper knowledge of the subsurface
configuration, this project has as its aim, the utilization of the results for the
sinking of bore holes.
The scope of work intends carrying out extensive geophysical (electrical
resistivity) survey within the study area, rudimentary geological mapping,
literature review and computer interpretation which will inform the
recommendation to be adopted with respect to bore hole citing.
1.4 GEOLOGYOF THE STUDY AREA
Ayanra village is located within latitudes N 07° 30’ and N 07⁰ 26’ and
longitudes E 06⁰ 53’ and E 06° 00’. The town is situated along the Auchi road at
the Southern part and Ikhakumo towards the North. The major problem of the
6. 6
study area is its lack of sufficient and safe water supply for domestic uses. The
major source of domestic water for the inhabitant in the study area is from both
hand-dug well and a stream at Oshunba. However, this stream is polluted by
activities of the local farmers especially during the dry season for fermentation of
cassava, washing of melon, clothes, passing of feaces resulting in unhygienic
nature making it unsafe for drinking and domestic uses. The area is located in the
Northern part of Edo
State in Nigeria and the rocks here belong to the crystalline basement of
South Western Nigeria. The area is composed generally of low lying Basement
rocks. The area is underlain by ferruginised sandstone, quartz, rich sandstone, rich
sandstone (non- ferruginised) and clay stone. This clastic sediment underlies
migmatitegneiss Basement Complex. The south western basement complex is one
of the three Basement Complexes in the country, the other two are the north central
and the South Eastern Basement complexes. The south western basement complex
of Nigeria lies to the rest West African in late pre-cambrian region to early
Paleozoic orogenesis. It extends westward and continues till Ghana. The basement
complex like the other two basement complexes has two major group of rocks.
These are;
1. Migmatite-gneiss complex which comprises biotite and biotite horn blend,
gneisses with intercalated amphibolites and
7. 7
2. Slightly migmatised, layered, medium grained granite and gneiss.
1.4.1 VEGETATION:
The area is part of the tropical rain forest belt of the South Western Nigeria.
There are two seasons, the rainy season which begins in May and ends in October
and the dry season which runs from November to April. The forest has been
drastically reduced due to persistent farming and bush burning. The area is of rural
setting and the inhabitants practice peasant farming. They grow cash crop mostly
cocoa and palm produce. Some practice mixed cropping such as yam, cassava,
melon, maize, pepper, beans, onions, and vegetables. Some of them engage in
fishing as part time job from the river available in the village.
8. 8
Fig .3:geologic map/ locationof VES point of the study area
1.4.2 Groundwateroccurrence in the study area:
The hydrogeologic settings of the area is a typical of any Basement Complex
terrain and groundwater in such terrain is usually found in two situations
(Bannerman and Ayibotele 1984): E.Y. Mbiinibe et al, Continental J. Earth
Sciences 5 (1): 56-63, 2010. Fractured poorly decomposed or fresh rock overlain
by a relatively deep zone of well decomposed rock and the fractured rock
9. 9
Groundwater is known to be more promising within granular alterite and the
transition zone immediately overlying the fresh bed rock (Chilton and Smith
Carrington 1984). In the study area, groundwater was identified to occur within the
weathered mantle developed on the crystalline rocks mainly migmatite, The zone
of weathering is relatively regular within VES 1 and VES 4 and slightly irregular
as confirmed by the variations in depth to bedrock which varies from rocks having
experienced prolonged weathering and tectonism which has given rise to thick
weathered mantle of 11-13m.
Fig.4:Map showing the ground water province of Nigeria
10. 10
1.4.3 Hydrogeologyof study area:
The hydro geologic settings of the study area are typical of any Basement
Complex terrain. Usually, in hard rocks, storage of water depends mostly on the
total thickness of the weathered and fractured zones and the yield here compared
with that of alluvial and sedimentary area is very small (R.K. Verma 1950).
Aquifers are formed in these hard rock areas from weathered and fractured zones
with the extent of weathering being depended on the presence of fractures at depth
and surficial morphological features. Thus, the geology of the area suggests good
hydraulic characteristics in terms of groundwater storage in the weathered zone.
Fissures on fresh rock joints tend to close at a depth of about 70m below which
there will practically be a limited circulation of ground water (M’ Kireld ,1950).
Isolated water may form below reservoirs mainly within fractured rocks and
pockets of weathered rocks with varying porosity and permeability of these
isolated reservoirs resulting in widely variable yields. The main source of recharge
in the area is through precipitation during the wet season. The main
hydrogeological unit in the area is the weathered zone.
11. 11
Typical thickness of various layers in hard rock is given below:
Layer No RockType Thickness (M)
1. Top soil 1 to 2
2. Weathered layer 10 to 20
3. Semi weathered layer/fractured 10 to 20
4. Hard rock Up to infinity
Fig.5:showing typical thickness of various layers in hard rock
Interracial and fracture porosities are common in weathered rocks in which
the clay are present as a result of the feldspar content in these rocks, thereby
reducing the permeability to some extent. Fracture porosity is common in jointed
and fracture rocks and these rocks are able to yield sufficient quantities of water to
meet the needs of a small community.
1.5 GEOPHYSICAL METHODS IN HYDROGEOLOGICALSTUDY
In the area of ground water study, the utilized methods include:
1. Gravity method
2. Electrical resistivity method
3. Seismic refraction method
4. Electrical self- potential method
12. 12
Of these five (5) methods listed above, the most commonly used, especially
for detailed exploration are the seismic refraction and electrical resistivity methods
(Koefed 1979, Telford et al 1976,) Lennose 1962,Vanderbeghe). However, the
most electrical resistivity method employing the Schlumberger array are used for
the study due to its low cost of field operation, its ability to detect local
inhomogeneities and its ability to investigate the change in formation resistivity
with depth.
1.6 PREVIOUS WORK
The literature work dealing with evaluation of ground water potential in the
weathered zone of the crystalline basement is diversified, some works in
weathering profiles are available (e.g. Oviei 1969, Renva 1964). Hydrogeologists
have been able to understand the occurrence of ground water in regoliths
(Omosinbola 1950). It was noted that most of the aquifers in the regolith of the
crystalline basement rocks are mostly of the perched type caused by irregular
weathering pattern of the rock.
13. 13
CHAPTER TWO
METHODOLOGY
2.1 RESISTIVITYMETHOD:
The electrical resistivity method employed in this study is the Schlumberger
array configuration . Electrical prospecting makes use of a variety of principles,
each based on some electrical properties or characteristics of the materials in the
earth (Egbai and Asokhia, 1998). In this method, measurements were made with
increasing separation between the electrodes about the midpoint. The instrument
used for data acquisition was the ABEM 3000 SAS Terrameter having an inbuilt
booster. This equipment has the ability of computing and displaying the apparent
resistivity on the subsurface with the input data of the current electrode AB and
potential electrode MN separation. There are a lot of geophysical methods which
use measurement of voltages or magnetic fields associated with electric currents
flowing in the ground. The current may be natural but are more often artificially
generated by direct contact or electromagnetic induction. Two types of arrays are
in common use, the Wenner array, and the Schlumberger array. Arrays can be use
for either profiling and/or depth sounding, often refer to as electric trenching or
drilling respectively.
Electrical resistivity surveys are used routinely in mining, coal, geothermal,
engineering application, hydrogeological investigation (Zoldy, 1964, Al’pon et al
14. 14
1966, Kienetz 1966). They are also use in oil and gas exploration (Keller, 1968,
Eadie, 1981, Spies, 1983). Their relatively recent use for sensing buried wastes and
waste mining is documented in Stoilar and Roux (1965). For the purpose of this
investigation, however, Vertical Electrical Sounding method of electrical
resistivity survey was adopted to investigate the electrical properties of the ground
in vertical discontinuities.
2.2 PRINCIPLES OF RESISTIVITYSURVEY:
The resistivity sounding method was first adopted by Conrad Schlumberger
(1912). This method involve the introduction of artificially generated current to the
ground. The generated potential differences are measured at the surface and
subsequently compared to the pattern of potential differences expected from
homogenous ground. The interpretation of the measurement is based on the
assumption that the subsurface consists of a sequence of distinct layers of finite
thicknesses, each of these layers is assumed to be electrically homogenous and
isotropic and the boundary between subsequent layers are assumed to be
horizontal.
These assumptions present only a very ideal description of the real
conditions that exist in the subsurface. The nature of the deviations of the real
15. 15
subsurface conditions provides information on the form and electrical properties of
subsurface inhomogeneities.
The resistivity of a material is defined as the resistance (R) in Ω between the
opposite face of the unit cube of the material (Keary and Brooks). For a cylinder,
the resistivity is usually represented by (𝜌). The resistance (R) across a unit length
(L) of cross sectional area (A) is expressed as
𝜌= R.A
/L (Ohms)……………………………………...1
Resistivity (𝜌) becomes apparent resistivity (𝜌a) when there is a deviation in
the assumption of homogenous materials to inhomogenous materials. All
resistivity measurements in general use require the measurement of resistance (R)
and the geometric factors used to calculate the apparent resistivity (𝜌a) can be
calculate from the first principles. Consider a current passing through homogenous
materials such as a cylinder, it will cause a potential drop (-𝛿v) between the end of
the element. The current in a conductor is generally equal to the voltage across it
divided by a constant; the resistance. This principle is known as Ohms law. The
resistance (R) is measured in Ohm when current (I) is in amps and voltage (V) is in
volts. Ohms law is related mathematically
V = IR……………………………………………………..2
Substituting equation 2 into 1
𝜌.L = V/I. A
16. 16
𝜌I/A = V/L. 𝜌I……………………………………………………………..3
Where V/L = potential gradient through the element in volt m-1 and I = current
density in amps m-2.
But consider a single current electrode on the surface of a medium of
uniform resistivity (𝜌). At the far end is the current sink away from the electrode.
The current, flow in form of a hemisphere shell away from the electrode centre at
the source of a distance, r. Therefore the surface area is 2πr2, so the current density
(I) is given by
i = I/2πr2………………………………………………..4
From equation (3), the potential gradient (v) associated with the current density (i)
is given by
V/r = -𝜌i = 𝜌I/2πr2
17. 17
Current flow line
v
equipotential surface
Fig. 6: showing current flow from a single surface electrode
Note that the minus sign only indicate that the current is acting / flowing in an
oppositedirection.
By integration with respectto V and r
𝛿v = 𝜌I/2πr2 𝛿r
∫ =𝜌.I/2π∫1/r2
V = 𝜌I/2π [r-2+1 / -2 +1]+C
V = 𝜌.I/2πr…………………………………………………………………5
Equation (5) allows us to calculate the potential at any point on or below the
surface of a homogenous half space. The potential gradient across electrode (C1
and C2) will be
Vp1 = 𝜌I/2π [1/r1 – 1/r2]……………………………………...6
𝛿v
v
18. 18
Where r1 = distance from current electrode C1 to potential electrode P1.
R2 = distance from potential electrode P1 to current electrode C2,
Similarly,
Vp2 = 𝜌I/2π [1/r3 – 1/r4]…………………………………....................7
Where r3 = distance from currentelectrode C1 to potential electrode P2.
R4=distance from potential electrode P1 to currentelectrode C2.
Therefore the potential differenceacrossthe circuit is
𝚫V = Vp1 – Vp2
𝚫V = 𝜌I/2π [1/r1 – 1/r2] – [1/r3 – 1/r4] is a function of the
electrode separation and it is a measure of the amount of earth that
contribute to the resistivity while the 2π represent the half space
covered by the circuit.
Thus, 𝜌a = 𝚫V/I. 2π/ [1/r1 – 1/r2] – [1/r3 – 1/r4]. Hence,
𝜌a is the apparent resistivity, it enables us to determine the change that
occur in the character of the surface where inhomogenities exist, as the
electrodes which are arranged on a line and then separation is increased
in a systematic manner with increasing depth of penetration.
19. 19
2.3 ELECTRODE CONFIGURATION:
This has to do with the manner in which the electrodes are arranged in
conducting an electrical resistivity survey.
These include the Wenner array which is widely used, with a vast amount of
interpretational literature; Two electrode array; gradient array used principally in
reconnaissance work, Dipole-Dipole (Eltran) array; popular in induced polarization
work, Pole-Dipole array; Square array; Multi-electrode array, and the
Schlumberger array; the only array to rival the Wenner in availability of
interpretational material. The Schlumberger array, in which only two electrodes are
moved, which is often preferred for speed and convenience was adopted in this
study. Consider a situation where the current sink (P1 and P2), which is the
electrode at a finite distance from the source (C1 and C2), which is the positive
electrode.
20. 20
C1 P1 P2 C2
L L
Fig.7:showing Schlumberger Electrode Configuration
2.4 SITE SELECTION:
Site selection is extremely significant in all sounding works especially in the
Schlumberger array, which is very sensitive to condition around the closely spaced
inner electrodes. A location where the upper layer is very inhomogenous is
unsuitable for an array centre. Directions of expansion are often constrained by
topography. There may be only one direction in which electrodes can be taken in
sufficient distance in a straight line. Also, paved environment is not suitable
because it affects the conductivity of the electrodes.
In choosing the site of resistivity sounding measurement and in particular
positioning of the potential electrode in this study, adequate attention was given to
the erroneous effect of near surface inhomogeneities upon the measurement, such
I
V
21. 21
as roads, ditches, wire fences, and buried metallic object like pipelines. Therefore,
if an inhomogeneity occurs close to the potential electrode, its effect is to alter the
potential difference measured.
Based on the above precautions, a total of four (4) evenly spaced VES site
were occupied in this study. In choosing the VES site in this study, it was ensured
that there was at least 150m of cleared straight line on both sides.
2.5 FIELDWORKAND EQUIPMENT:
Resistivity measurement were made using the Schlumberger array which
consists of two sets of electrodes, potential electrodes and the current electrodes
arranged in a straight line with a fixed point of array. Each of the electrodes
consists of metal stakes driven into the ground by hammer. Each of the electrodes
is then connected to ABEM AC Terrameter (ABEM SAS 3000), with cables made
of flexible multistrand insulated wires of several hundreds of metres in length.
The purpose of resistivity sounding is to investigate the change of the
formation resistivity with depth and this can be achieved by changing the distance
between the current electrodes, so that the depth range to which the current
penetrates is changed. Measurements were carried out such that there are six
equally spaced points on a decade of a log scale. The end result of the field
22. 22
measurement is the computation of resistivity values and the thicknesses of the
layers. These are then plotted on a log-log paper.
Resistivity survey requires instruments and some means of making contact
with the ground, such as cables and electrodes. For the purpose of this survey,
metal bar electrodes were used. The cables used in resistivity measurement are
normally single core, multistrand copper wires insulated with PVC. The thickness
is usually dictated by the need for mechanical strength rather than low resistance.
The contact resistance which is the major limitation on current flow depends on
moisture and contactarea.
The source of AC current for this survey was the ABEM SAS 3000,
TERRAMETER. This is a resistivity meter with reasonably high sensitivity. The
equipment is strong, potable and easy to use. This instrument has high penetration
capability (0-600m), which makes it suitable for subsurface investigation. It is also
very accurate to the tone + 3-10% for readings as low as 0.01 – 0.001 ohms.
23. 23
Fig.8:showing the equipment used in acquiring VES in a given location
2.6 PRECAUTIONS:
Precautions are of great significance in any geophysical work. These were
strictly adhered to during the cause of the data acquisition and they include the
following:
1. The effect of lateral inhomogeneities close to the potential electrodes has to be
considered; the effect is to alter the potential difference measured.
CABLE
ELECTRODE
TAPE GPS ABEM TERRAMETER
24. 24
2. Current cables must never be connected to or disconnected from the electrodes
while the current source is switch-on.
3. Grasses are cleared around the electrodes to prevent current leakage.
4. Stringent safety precaution was also observed in the whole length of the
current cables for passers-byand livestock.
5. Care was also taken not to allow the cable to become tangled, which can cause
permanent kinks.
2.7 DATA ACQUISITION:
A total of four (4) VES readings were taken using a terrameter and two sets
of electrodes; potential and current electrode arrange in a straight line with a fixed
point of array. The first point of consideration when using the schlumberger array
is that of station. The sounding stationing has to be sited on a long and straight
stretch of hand on a flat terrain, so as to minimize error of measurement and
interpretation. Limited separation of 0.2m was used for the potential electrodes and
this was increased on when it became too small for reliable reading. On the whole,
a total of four VES were made with half current electrode separation (AB/2) from
1.0 to about 147m. At least two readings were taken with the same values of AB as
the MN values were gradually increased. The sequence was 1.0, 1.47, 2.15, 3.16,
4.48, 10.0, 14.7, 21.5, 31.6, 46.8, 68.1, 100, 147 metres. This increase gives good
25. 25
sampling intervals on a logarithmetric plot. Apparent resistivity values were
calculated by means of the usual constants based on the schlumberger electrode
array and plotted in double logarithm paper against half electrode separation AB/2.
The readings from the field data were illustrated as electrical resistivity sounding
curves. These curves represent the changes of apparent resistivity (𝜌a) as a function
of half current electrode separation.
2.8 REDUCTION OF FIELD DATA:
The current electrode and potential electrode are spaced away from each
other, with the potential electrode at a fixed point about a center position in exact
log spacing sequence. As the current electrode distance increases, the potential
difference reduced, until point is reached where the voltage-drop becomes too
small to be exactly measured and thus, the potential electrode has to be moved
further apart to a distance such that a fixed ratio is maintained.
The resulting sounding curve derived by plotting 𝜌a (Ω-m) against AB/2 (m) at
location on a logarithmic sheet will thus, consists of a number of separate segments
(fig.9). The reasons for these are not far-fetched. In the first place, measurements
are made with a symmetrical electrode configuration in which the ratio of the
potential electrode to current electrode has a finite value, changing the distance
between the measuring potential electrode tend to alter this ratio and thus, alter the
26. 26
apparent resistivity which is dependent on it. The other reason is attributable to the
existence of near surface inhomogeneities which affect current distribution pattern
and the current density, thus reflecting in the resistivity measurement (Koefoed,
1979).
For an easy interpretation to be made, the segments of the curves has to be
join by moving all the segment parallel to the resistivity axis so that a continuous
curve is formed. To do this, overlap reading must be made. Ideally, there should be
at least three such at each change-over but two are more usual and one is
unfortunately the norm (J. Milson, 1989).
By multiplying all the apparent resistivity (𝜌a) values at the beginning or end
of each segment by a constant factor depending on whether the segment is to be
raised if the sequence is increasing downward or dropped if the sequence is
decreasing downward. If the segment is to be raised, the constant factor is obtained
by dividing the higher apparent resistivity values with the lower values. While to
drop the higher apparent resistivity values of the segment, the constant factor is
obtained by dividing the lower value by the higher value at the point of change-
over, then, multiply all values below it by the constant factor.
27. 27
100
𝜌a (Ωm)
10
10 100
AB/2 (m)
Fig.9:Diagramshowing unadjusted curve
With the procedure above, it is assumed that the inhomogeneities is small
compared to the distance between the current electrode. Hence, the current-field
that would exist if the inhomogeneities were absent is very nearly homogenous in
horizontal direction. The linking removes the shift or jump in the curve, thus a
smooth curve is formed which can be interpreted by matching it with master curves
along side the auxiliary curves as recommended by Orellana and Moorney (1966)
or by a suitable computer program.
In this study, all the field curves were subjected to the smoothing procedure
described above and were later followed by computer assisted iterative
interpretation procedure as described by Zohdy, 1989.
28. 28
2.9 INTERPRETATION PROCEDURE:
There are basically two methods of interpreting geo-electric data sounding
data.
1. The time consuming traditional method of interpretation; such as auxiliary
point technique (Zohdy, 1965.) or curve matching procedure using albums of
theoretical curves (Orellana and Mooney, 1966)
2. The other is the direct iterative computer assisted interpretation method.
The auxiliary point method, first published by Ebert (1943) involves
matching a small segment of the plotted curve with families of master curves (two
layer curves) and auxiliary curves (three layer curves) having equal modules as the
plotted curves. There are four types of curve which are employed in this
interpretation. These include the Ascending or A-type curve, the Descending or Q-
type curve, the Bowl shaped or K-type curve and the Bell shaped or H-type curve
which is the predominant in the case of this project. The A and Q-type layer curves
are two layer curves while the H and K-types are three layer curves.
The acquired data is plotted on a transparent log-log paper with 𝜌a on the Y-
axis and AB/2 on the X-axis. The transparent log-log paper is then superimposed on
the families of the master curves such that the co-ordinates of the master curve and
the plotted curve are parallel. One sheet is moved relative to the other, keeping the
29. 29
vertical axis parallel until a segment of the field curve fits one of the families of the
master curve.
The computer assisted interpretation technique in resistivity sounding is
based on a 9-point digital lineal filter method of Ghosh (1970) to compute the
theoretical resistivity curve for a given set of layer parameters, or a 20-point digital
filter of O’Neil’s for models with layer parameters having resistivity contrast of <
1/25 for any two cons layers (Koefoed, 1979). Two stages are involved in
computational interpretation. The first stage involves the computation of the
“resistivity transform” of the sample values from the layer parameters. This is done
by the use of the Perekis Recurrence Relation given as;
Ti = [Ti + 1 + 𝜌 tan h (𝜆ti)]/[1 + Ti + 1 tan h (𝜆ti)/𝜌I]
Where I= 1, 2, 3,…….ni; denotes the number of subsurfacelayers.
Ti= resistivity transform correspondingto the ith layer.
Ti and 𝜌I= thickness and resistivity of the ith layer respectively.
The second stage involves using suitable computer program to evaluate the
equation given above; computed 𝜌a values were obtained for each measurement.
And by the process of “trial and error”, the model parameters were adjusted to
attain a good match between the field curves and the computed theoretical curves.
30. 30AB/2
AB
2
𝜌a
Fig. 7 Showing .10:types of
curves
AB/2
Pa
𝜌a
𝜌a
AB/2
H –CURVE OR BOWL
SHAPE
K – CURVE OR BELL
SHAPE
A – CURVE OR
ASCENDING
Q – CURVE OR
DESCENDING
31. 31
CHAPTER THREE
3.1 RESULTS OF DATA ACQUISITION INTERPRETATION
Results and interpretation of the soundings generated from the study area are
presented below as field and computed data, layer earth model and plot of apparent
resistivity (ρa) vs current electrode spacing (AB/2) and from which geo-electrical
sections were drawn. All the curves fall within the H or bowl type of Kalenov
classification (1957).
The observed field data were used to produce depth sounding curves. The
qualitative interpretation of field sounding curves were subjected to partial curve
matching techniques using two layer apparent resistivity curves. The sounding
curves were obtained as a result of plotting the apparent resistivity values from the
field work against electrode spacing. The results of the curved matched values
were iterated using the resist software (Vander Velpen, 1988). The computer
modeling utilized the quantitative Interpretation (curve matching) result to obtain
the layer resistivities and Thicknesses of the subsurface under investigation. This is
shown in the table below:
34. 34
3.2 RESULT OF VES INTERPRETATION
VES1
It shows an H or (Bowl) shape ascending type curve. From the model, there are
five interpreted geo-electrical sections. The first geo-electrical layer (GL1)
corresponds to the top soil which has a resistivity value of 169.59Ωm with a
thickness of 0.88m. The second and third layers (GL- 2 and GL-3) with resistivity
values of 36.543Ωm and 86.707Ωm and thicknesses of 2.3620m and 4.0909m
represent the clay, sandy clay layer. The fourth layer (GL- 4) which has a
resistivity value of 162.29Ωm and a thickness of 11.352m is interpreted as the
(sand layer) weathered zone. The fifth layer (GL-5) is interpreted as the fresh
Basement with a resistivity value of 4756.4Ωm and an infinite thickness.
38. 38
RESULT OF VES INTERPRETATION
VES2
This shows an H type curve. There are six geo-electrical layers from the model.
The first geo-electric layer (GL-1) with resistivity value 451.91Ωm with a
thickness of 0.48617m which is the top soil. The second and third geo-electric
layers with resistivity values of 17.642Ωm and 15.298Ωm has thicknesses of
0.86214m and 1.75m is interpreted as the clay layer. The fourth geo-electrical layer
with resistivity value of 344.10Ωm and thickness of 3.3615m represent the
silty/sandy layer or the slightly weathered zone. The fifth geo-electrical layer with
resistivity value of 680.29Ωm and thickness of 6.51m is interpreted as the
weathered or fractured zone. The sixth geo-electrical layer is the fresh Basement
and also the last layer with resistivity value of 4172.2Ωm and an infinite thickness.
41. 41
RESULTS OF VES INTERPRETATION
VES3
It is an H type of curve with five geo-electrical layer based on the modeled layer.
The first geo-electrical layer represents the top soil with resistivity of 120.47Ωm
and thickness of about 0.755m. The second and third geo-electrical layers are
interpreted to be the clay/silty layer with resistivity values of 64.508Ωm and
21.096Ωm with thicknesses of 1.1007m and 1.4618m. The fourth geo-electrical
layer with resistivity value of 863.97Ωm with thickness of 2.9612m is interpreted
as the weathered zone or fractured layer. The fifth and the last geo-electrical layer
has resistivity value 7203.7Ωm with an infinite thickness and it represent the fresh
Basement.
44. 44
RESULTS OF VES INTERPRETATION
VES4
It displays an H type of curve. There are five geo-electrical layers based on the
modeling. The first geo-electrical layer is the topsoil with resistivity of 366.10Ωm
and thickness of 0.58m. The second geo-electrical layer has resistivity of
126.02Ωm and a thickness of 2.17m which is interpreted as the clay sand layer.
There is a drop of resistivity value which is 88.476Ωm with a thickness of 4.85m
indicating a clay silty layer. The fourth geo-electrical layer represents the
weathered layer with resistivity value of 789.51Ωm a thickness of 12.96m. The last
layer is the fifth with resistivity value of 1737.7Ωm with an infinite thickness
represents the fresh Basement.
45. 45
CHAPTER FOUR
DISCUSSION
4.1 DISCUSSION OF THE RESULTS:
To study the possible variation of the subsurface in Ayanra for the exploration of
water, a total of four VES were measured and interpreted. To this end, contour
maps were generated. They are as follows;
1. Overburden thickness
2. Thickness of the fractured zone
3. Basement Elevation
4. Surface Elevation
46. 46
Fig.11:showing the overburden thickness
OVERBURDEN THICKNESS
From the overburden contour map, the green colour indicates the regions
with the thickest overburden while the red colour indicate the region with thin
overburden, also known as the danger zone. The central region of the map is
dominated by the red colour thus, it represent the thinnest or shallowest region and
it is the least productive in terms of water prospecting. While the green colour
47. 47
dominates the edge of the map thus, representing the thickest region of the map.
The importance of this map is to delineate the cut-off limit where water can be
drilled. From the map, it is evident that VES four with the thickest overburden
(7.34m) with respect to its fractured zone thickness, is the most productive.
Followed by VES one (7.33m) and then VES two.VES three with a very thin
overburden (3.32m) is a dangerous zone and should be ignore in other to cut down
costduring exploration.
Fig.12:showing the fractured zone
48. 48
THICKNESS OF THE FRACTURED ZONE
This is the most significant when exploring for groundwater in a Basement
Complex as it indicates how productive a particular area is going to be with respect
to the other area by looking critically at the thickness values alongside its
overburden thickness thereby reducing the cost of drilling unnecessary amount of
boreholes. And it also help to decipher the type of drilling that should be
undertaken and the most suitable location for such exercise. From the fractured
zone map shown above, it is obvious that the most productive weathered zone is
located at VES four, followed by VES one and VES number two and this are
represented as the blue region on the map while the region coloured red indicates
the least productive zone due to how thin the layer is when compared to its high
resistivity value of 863.97Ωm. It is advisable that a total of two boreholes can be
drill through a depth of 11m – 13m at location four and one respectively.
49. 49
Fig.12:Map showing surface Elevation
SURFACE ELEVATION
Water will normally flow from a region of high topography to a region of
low topography and this is evident from the above map as water will be expected
to flow from VES location four with the highest elevation to VES one location
probably due to the high rate of fractural connectivity as a result of secondary
porosity and unable to flow to location three due to massive blockage of
unweathered granitic rock with a very poor connectivity. This result also supports
50. 50
the high prospect of location four and one. The presence of a stream along VES
two and four also show how water can take advantage of topography i.e. the river
will not be able to flow to a higher elevation (location two) thus flowing to location
four and through it also flow to location one whose fractured zones have a very
good connectivity with that of VES four. This also applies to the rainy seasons.
Meaning that location two may not be able to hold water for a very long time due
to its position.
51. 51
Fig.13:Map showing the BasementElevation
BASEMENTELEVATION
Due to the several tectonic events that may have taken place in a specific
Basement Complex, the basement elevation might have been altered for a good
number of times which could have resulted to what we have in the above map.
From the map above, we can see here again that VES four with the highest
basement elevation is also shown to be the most productive. This may not always
accurately correspond to the above maps due to the effects of tectonism which
52. 52
could have been a fault displacing a formal high location to a lower position, but to
a large extent it does support the results of the other maps and still show VES four
as the most productive.
A summary of the VES data interpreted according to the modeling is shown below.
VES
No
NORTHINGS
Deg min Sec
EASTINGS
Deg min Sec
Type
Description
Modeled
Layer
No
Overburden
Thickness
Fractured
Zone
Thickness
Basement
Elevation
Surface
Elevation
1 7 28 48.5 5 57 47.
6
H 5 7.33 11.35 324.67 332
2 7 29 01.0 5 57 31.
8
H 6 5.75 6.51 309.31 315
3 7 28 53.9 5 57 47.
9
H 5 3.32 2.96 325.68 329
4 7 28 58.2 5 57 51.
6
H 5 7.34 12.96 334.66 342
Figure.14:Showing the Interpreted Geo-electric Parameters
53. 53
CHAPTER FIVE
SUMMARY AND CONCLUSION
Geophysical survey methods are now widely used for the investigation of
the subsurface geology. The electrical resistivity techniques carried out in Ayanra
is to investigate the nature and distribution of groundwater in weathered zones.
From the interpretation, different sections of the subsurface geology in the
Basement terrain (area studied) were revealed and made known including the
target zone (fractured zone) where groundwater occurs. This zone of interest is
known to exist at a depth range of 7.3 to 7.4m with a thickness of about 11to13m.
Therefore, it is advisable to drill the first borehole at VES 4 location because it has
the thickest overburden and productive window. At least two boreholes should be
drilled at a depth of 7-8 m. Electrical resistivity survey is very fast and the
equipment used in carrying out the operation is relatively cheap and easy to
operate when compare to other geophysical field method. Results of this study
have gone to some extent to prove that electrical survey is a practical tool for
obtaining significant geological subsurface information.
54. 54
5.2. RECOMMENDATION
I recommend electrical resistivity method as an effective geophysical
approach to investigating groundwater distribution in Basement rocks before
proper borehole drilling is done, since it’s cheaper and safes time among other
known geophysical methods.
55. 55
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